The present invention relates to a method and apparatus for detecting objects by the use of passive infrared sensors. The invention is especially useful for detecting intrusions through a door or window, and is therefore described below with respect to such an application, but as will be pointed out below, the invention could be advantageously used in many other applications as well
Passive infrared (hereinafter xe2x80x9cPIRxe2x80x9d) sensors are widely used for detecting infrared (IR) radiating bodies, such as a person, particularly for detecting intrusions in monitored spaces. Where the ambient temperature is less than body temperature (e.g., above 36xc2x0 C.), the radiation generated by the intruder can be sensed to trigger an alarm.
There are many applications, however, where it is desired to monitor a limited space, such as the region adjacent to a window or door of a room, in order to detect an intrusion through the window or door, but to ignore movements in other regions within the room. In such applications, it is necessary to install the PIR detector system very precisely in order to make it sensitive only to movements within the region to be protected and to make it insensitive to movements outside that region. In addition, it is also frequently necessary to use a PIR detector internally of the room and another one externally of the room in order to enable the system to detect movement in the xe2x80x9chostilexe2x80x9d direction, (i.e., through the window or door into the room) and to make it insensitive to movements in the friendly direction, (i.e., within the room or towards window or door).
PIR detectors are also used in other applications, such as for controlling automatic doors.
One object of the present invention is to provide a method, and also an apparatus, for detecting objects in a manner which enables the direction of movement of the object to be easily determined. Another object of the invention is to provide a method, and also an apparatus, for detecting intrusions, such as intrusions through a door or window of a room, in a manner which ignores movements in other adjacent regions.
According to one aspect of the present invention, there is provided a method of detecting intrusions with respect to a monitored space comprising: exposing the monitored space to a passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; generating a movement signal consisting of a positive polarity signal and a negative polarity signal when both have been generated within a first time interval such as to indicate the movement of an object within the monitored space; determining from the relative sequential order of the positive polarity signal and negative polarity signal in the movement signal the direction of movement of the detected object, and particularly whether the movement direction is a hostile direction or a friendly direction; and actuating an alarm when the direction of movement of the movement signal is determined to be in the hostile direction, but not when it is determined to be in the friendly direction.
According to further features in the described preferred embodiment, the method further comprises disabling the actuation of the alarm during a second time interval after a generated movement signal has been determined to be in the friendly direction. More particularly, the disabling of the actuation of the alarm is effected by applying each movement signal, after a short time delay, to restart the second time interval during which the actuation of the alarm is disabled.
As will be described more particularly below, such a method actuates the alarm upon detecting a movement In the hostile direction (i.e., from a door or window into the interior of the room), but will be insensitive to movement in the friendly direction (i.e., from the interior of the room towards the window or door). Moreover, each time a movement has been detected, the time interval for disabling the alarm, if not already actuated by a hostile movement, is restarted so that the system will be insensitive to any movements during this time interval, whether in the friendly direction as well as in the hostile direction.
In the described preferred embodiment, the first time interval during which a positive polarity signal and a negative polarity signal must be received before a movement signal is generated, is measured in seconds, e.g., preferably about 4 seconds, which is a reasonable time to assume that both those signals were generated by the same moving object. The second time interval, during which the alarm is disabled after the generation of a movement signal (assuming the alarm has not been actuated by a hostile direction movement) is measured in tens of seconds or minutes, e.g., 40-60 seconds, which is a reasonable time to assume that no intrusion will be attempted through a door/window after a friendly body has been moving in the room. The short time delay imposed on each movement signal for restarting the disabling interval (the second time interval) is a small fraction of a second, e.g., 30 msec., merely to provide enough time for a detected movement in the hostile direction to actuate the alarm before the alarm can be disabled. Once the alarm is actuated, it continues to operate according to the alarm circuit, e.g., intermittently or for a predetermined time interval.
As briefly described above the invention is also applicable in other applications wherein it is necessary or desirable to detect not only the movement of an object within a monitored space, but also the direction of movement of the object. Such applications could include the controlling of automatic doors so as to automatically open the door when a person approaches the door, to automatically close the door when the person is moving away from the door, and to make the door control insensitive to movements parallel to the door. Another possible application of the invention would be to count traffic moving in each direction.
According to another aspect of the present invention, therefore, there is provided a method of detecting movement of an infrared-radiating object in a predetermined direction within a monitored space, comprising: exposing the monitored space to a passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; generating a movement signal constituted of a positive polarity signal and a negative polarity signal when both have been generated within a first time interval such as to indicate the movement of an object within the monitored space; and determining from the relative sequential order of the positive polarity signal and negative polarity signal in the movement signal the direction of movement of the detected object.
According to a still further aspect of the present invention, there is provided apparatus for detecting moving infrared-radiating objects in a monitored space, comprising: a passive infrared sensor to be mounted to view the monitored space, the passive infrared sensor having a first sensor element generating a positive polarity signal when its field of view senses an infrared-radiating moving object, and a second sensor element generating a negative polarity signal when its field of view senses an infrared-radiating moving object; a time window circuit for receiving the generated positive polarity signals and negative polarity signals and for generating a movement signal when a positive polarity signal and negative polarity signal have been generated within a first time interval, such as to indicate the movement of an object within the monitored space; and a direction-determining circuit for determining, from the relative sequential order of the positive and negative polarity signals in the movement signal, the direction of movement of the detected object.
According to further features in the preferred embodiment of the invention described below, the apparatus is particularly useful for detecting intrusions and further comprises an alarm; and an alarm actuating circuit which actuates the alarm when the direction of movement of the movement signal is determined to be in a hostile direction, but not when it is determined to be in a friendly direction.
Further features and advantages of the invention will be apparent from the description below.